feat(ui): shared |bytes |rate |duration |ago filters and status_class from Settings → Thresholds (#5093)

#5093 "Shared format filters and status_class driven by Settings → Thresholds".

- steward/core/formatting.py: human_bytes (binary units), human_rate,
  human_duration (two largest units), ago (a <time> titled with the absolute
  UTC time) and status_class(value, kind, thresholds) -> ok/warn/crit/unknown,
  which raises on a kind the thresholds don't name.
- app.py registers them as the bytes, rate, duration and ago filters and the
  status_class global; threshold_style is now graded by status_class (and caps
  its weight at 500).
- Replaces the copies: docker's _human_bytes/_human_uptime, core alerts'
  _duration, the host-agent fmt_bps/fmt_bytes macros, the Traefik and UniFi
  byte-rate arithmetic, the swarm-failures ago macro and two uptime spans.
- Tests: a table per helper, and every template compiled against the app's
  filters, so an unknown filter fails CI rather than a page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/app.py

@@ -130,6 +130,17 @@ def create_app(
         lambda value, kind: _ts_for(value, kind, app.config.get("THRESHOLDS", {}))
     )
 
+    # One way to write a size, a rate, a span and a moment, and to grade a
+    # reading ('ok' / 'warn' / 'crit' / 'unknown') against the same cutoffs.
+    from .core import formatting as _fmt
+    app.jinja_env.filters["bytes"] = _fmt.human_bytes
+    app.jinja_env.filters["rate"] = _fmt.human_rate
+    app.jinja_env.filters["duration"] = _fmt.human_duration
+    app.jinja_env.filters["ago"] = _fmt.ago
+    app.jinja_env.globals["status_class"] = (
+        lambda value, kind: _fmt.status_class(value, kind, app.config.get("THRESHOLDS", {}))
+    )
+
     # Forged-request protection: every POST form renders csrf_field(), HTMX and
     # fetch() send csrf_token() as X-CSRF-Token, and one check refuses any
     # unsafe request without it (steward/auth/csrf.py).